Generally, solid oxide fuel cells operate at high temperatures in the range of about 750° C. to 1000° C. These high temperatures are challenging to the materials employed, and are of particular concern with regard to the stability of the anode structures. For fuel oxidation, the so far preferred anode material comprises metallic nickel. Nickel is also preferred for hydrocarbon fuel since it is a good catalyst for hydrocarbon reformation.
Nickel oxide cermet structures have been suggested as anode materials for SOFCs for a number of years. Ni-cermet anodes usually have a 3-phase structure formed by nickel particles, ceramic particles (typically yttria stabilised zirconia, YSZ) and pores which are formed during the manufacturing process. The ceramic component of the cermet provides the necessary mechanical strength of the structure. Each of the components of the 3-phase structure furthermore forms a continuous path throughout the entire anode structure so as to provide transportation of electrons, oxide ions and gas, respectively.
However, the suggested anodes do not withstand repeated redox cycling during operation for a longer time without mechanical failure, resulting in the degradation of the electrical cell performance. The degradation is initiated by a coarsening of the nickel particles that takes place by grain growth during operation. If the fuel gas flow is lost during operation, the nickel particles will be oxidised to NiO electrochemically or by air that may penetrate into the anode compartment. The volume increase that is associated with the Ni oxidation causes disruption and crack formation in the ceramic backbone and the electrolyte because there are always volumes in which the porosity is too small to accommodate the resulting volume expansion.
T. Klemmensoe, Charissa Chung, Peter Halvor Larsen and Mogens Mogensen demonstrated in the article “The mechanism behind redox instability of SOFC anodes” that the redox stability of the anode in small and medium scale SOFCs is considered important for safety reasons. The technological aim has been reported to be 5-20 cycles per year during the life time of the cell. The commercial life of 5 years thus equals to a total of 25-100 cycles. However, in the prevalent anode supported design, oxidation of the anode is known to be detrimental for the cell performance. The degradation of redox cycling is believed to be related to bulk expansion of the anode, yet the mechanism behind the process has not previously been investigated. It was further demonstrated that a high strength, as achieved by using zirconia with 3 mole yttria instead of 8 mole, decreased the expansion during oxidation of a Ni—YSZ cermet structure. The article was published in SOFC IX, S. C. Singhal and J. Mitzusaki, eds. PV 2005-07, The Elecrtrochemical Society Proceedings Series, Pennington, N.J., 2005.
U.S. Pat. No. 6,099,985 discloses an SOFC comprising an anode which is fabricated from ceria mixed with a nickel oxide/magnesium oxide material to stabilize the nickel against coarsening during high temperature SOFC operation. MgO advantageously forms a single phase with NiO, while showing limited solubility in zirconia and ceria.
However, while the addition of MgO reduces the coarsening of nickel particles to a certain extend, at the same time the difference of the thermal expansion coefficient of the anode layer and electrolyte layer is increased, thereby weakening the overall mechanical stability of the SOFC, especially during heating/cooling cycles.
US-A1-2003/0165726 relates to a structured body for an anode suitable for fuel cells, comprising a structure formed by macro-pores and an electrode material having two reticular systems which intergage. The first system is made of a ceramic material, such as zirconium oxide stabilized with yttrium (YSZ), aluminium oxide, titanium oxide, doped cerium oxide, magnesium oxide, and/or a spinel compound. The second system contains metals, for example Ni derived from NiO, to bring about electrical conductivity, and may further contain MgO as an inhibitor of grain growth. In order to obtain an anode structure, the particles of a ceramic material (e.g. YSZ) and of a metal oxide are put into sufficiently fine form for the formation of the reticular systems by grinding and classification. A homogeneous mixture in the form of a slurry is formed from the particles, the pore forming materials and a liquid. The slurry is cast to form a layer. The slurry is cast in an absorbent mould so that some of the liquid is removed from it. At the same time, a marginal zone arises in which a lack of pore forming materials is present, resulting in an inhomogeneous structure.
However, in US-A1-2003/0165726 an inhomogeneous structure is obtained in which two reticular systems intergage. Thus, the first reticular system comprises a ceramic material and other oxides, and the second reticular system comprises nickel oxide and MgO as a grain growth inhibitor. The oxides comprised in the first system do not interact with the nickel oxide of the second system during the sintering, contrary to the composition forming the anode support layer and/or anode layer of the present invention.
US-A1-2003/0235752 relates to a fuel cell assembly comprising nickel-based anodes. To prevent repeated anode oxidation, oxygen getter devices containing oxygen-gettering materials such as nickel foam, a nickel wire or a nickel mesh, are provided in the fuel passageways leading to and from the anodes. Oxidation of the oxygen-gettering materials is readily reversed through reduction by fuel when the assembly is restarted.
U.S. Pat. No. 6,048,636 discloses an electrode for a fuel cell which has a porous self-supporting layer and another layer with catalytic properties disposed on the self-supporting layer. Said self-supporting layer consists of a cermet comprising Al2O3 or TiO2, to which nickel is admixed (This relates only to a cell support and does not contain any ionic conducting material (Zirconia or Ceria).
WO-A1-2004/013925 relates to a material suitable for use in a solid oxide fuel cell, especially an anode thereof, comprising an optionally doped double perovskite oxide material, and further discloses a SOFC comprising said material.
US-A1-2003/0035989 relates to a SOFC which comprises a solid electrolyte comprised of an electronic insulator which allows transfer of anions, a ceramic metal composite anode and a cathode. In order to overcome the problems associated with the presence of complex organic sulphur compounds in a hydrocarbon fuel stream for use in a fuel cell without increasing fuel-processing complexity, a porous copper cermet or copper-nickel-alloy cermet is provided by obtaining a sintered nickel cermet, leaching at least a part of the nickel, thereby increasing the porosity of the cermet, and adding Cu back into the pore structure.
WO-A2-2004/030130 relates to a high temperature fuel cell system comprising an anode channel, an anode inlet and an anode outlet, a first anode channel portion proximal to the anode inlet, a second anode channel portion proximal to the anode outlet, and a gas separation means operable to enrich a first gas component of an anode exhaust gas exiting the anode outlet to produce a first product gas enriched in the first gas component. The first anode channel portion comprises an anode material that is resistant to carbon deposition and active for direct oxidation of hydrogen, and at least one hydrocarbon fuel or mixtures thereof. The second anode channel portion comprises an anode material that is catalytically active for steam reforming of at least one hydrocarbon.
However, while most of the suggested anode structures for a SOFC do not prevent coarsening of nickel particles at all, the proposed addition of MgO for coarsening prevention disadvantageously destabilizes the SOFC due to an increase of the thermal expansion coefficient differential between the anode and electrolyte layer.